Just hours after she hypnotised the Latitude Festival in Southwold, Californian harpist Joanna Newsom floated into London's grandest outdoor venue to sprinkle further fairy dust.

Newsom's concerts here have become increasingly intimate while the venues become ever more appropriate settings for her baroque music and extraordinary poetry. Last year she performed at the Barbican with the London Symphony Orchestra, then at the Albert Hall with a three piece band. Here, she was alone on harp and grand piano, her only accompaniment the ocasional chiming of the courtyard clock.

Knitting away at her harp she sang long, meandering songs that featured the occasional refrain but never something so obvious as a chorus, building mysterious worlds of words with a vocabulary broader than the average novelist, never mind pop singer.

Her voice, once a quivering squeak that caused more heads to ache than hearts to break, is becoming increasingly soulful. It now sounds well trained but still unique.

There are no immediate plans to follow-up her magnificent 2006 album Ys, a five-song, string-laden epic whose shortest track was more than seven minutes long. Nevertheless, she tried out a few new pieces this evening. One saw her picking out simple, low notes on the piano while her cryptic vocals gradually built in intensity. She claimed it was unfinished. It sounded perfect.

This wasn't quite the perfect union of artist and space. Clearly freezing in a gown that exposed her arms and shoulders, she never seemed completely comfortable. But frequently, as her bewitching words cast their delicate spell, even these impressive surroundings were quite forgotten.